The Applicant's prior German Patent Application P 41 22 430.2 which corresponds substantially to U.S. Ser. No. 896,949 filed Jun. 11, 1992, describes a device comprising a measurement tube serving as a measurement path, which tube connects the interior of the water-containing vessel to the ambient air. The measurement tube has an associated sensor (steam sensor) connected to a control device for influencing the heating power. An automatically controlled supply of heating power ensures that the amount of water evaporating at atmospheric pressure is exactly such that any residual air is expelled from the vessel. Between the hot steam leaving the vessel and the cold air entering from the outside an interface is formed, which is detected by the sensor and whose location is maintained constant by controlling the heating power within the measurement area. The formation of this interface is a sign that the so-called steam point is reached, when any residual air is expelled from the vessel. This steam point is exactly maintained by the control system. Thus, said sensor enables exact control with a minimal power supply and minimal liquid losses.
Moreover, cooking devices are known which control the power supply via a cooking-substance sensor, the sensor detecting the temperature of the cooking substance directly or indirectly and controlling the power supply via suitable electronic devices depending upon a selected function. The sensor may be introduced directly into the cooking substance, for example immersed in a liquid or constructed as a spit. Indirect measurement of the temperature of the substance being cooked is effected, for example, by detecting the temperature at the bottom of the cooking vessel or by incorporating the sensor in the cooking vessel.
With both systems the sensors are optimized for only one purpose. For example, the first-mentioned system enables foodstuffs to be cooked in water or in steam with minimal costs but roasting is not possible. Conversely, the second system, where the temperature of the food to be heated is detected directly or indirectly, allows roasting but the boiling point of a liquid cannot be detected exactly, so that such systems always operate with some "advance power", leading to increased loss of liquid and energy.
DE-OS 39 28 620 describes a cooker in which the power supply of at least one cooking element is controlled by a microprocessor. This enables the progress of the cooking process to be derived from the actual condition of the substance being cooked. One embodiment of the prior-an device combines a sensor which detects the temperature of the bottom of the vessel with an external detector which measures the temperature of the cooking substance. This combination makes it possible to realize a cooking program without burning or boiling-over of the substance to be heated.
Moreover, DE-OS 32 05 123 describes a device for cooking foodstuffs in the heating space of a microwave apparatus comprising a sensor arranged in the flow path of the air leaving the heating space. In order to enable food in the heating space of the microwave apparatus to be cooked or warmed up fully automatically a further sensor is arranged in the air inlet path to the heating space. To determine the temperature inside the food there is provided a detection device. Finally, for automatically controlling the cooking process or cooking cycle there is provided a programmable microcomputer which receives the data detected by means of the sensors and the detection device and which is loaded with various basic programs to control cooking processes for different categories of food.